He said: "I think Prince Charles does an amazing job for the country ... I think he's perfectly entitled to express his views and personally I find no problem with it at all. I think it is completely unreasonable not to expect that he has views or that he transmits them to government ministers. But they are not views that I have ever, ever regarded as party political. I personally don't think it has ever caused any difficulties for ministers."

When asked whether he thought the prince had moved into party politics Mr Blair joked: "I don't know if I can answer that question until I've had the focus group" - a reference to complaints made in the royal diary, disclosed this week on the orders of the judge, about ministers being unable to act without first consulting such a group.

The prime minister's intervention came as the prince's effort to obtain summary judgment against Associated Newspapers ended at the high court after more than 12 hours of legal argument. The newspaper group, which publishes the Mail on Sunday, is being sued for breach of confidence and infringement of copyright after it published extracts from a private journal Prince Charles wrote and sent to friends detailing his experiences at the UK handover of Hong Kong to the Chinese in 1997.

The decision of the judge, Mr Justice Blackburne, over whether the case should receive a full jury trial will not be known for several weeks.

But the move by the prince appears to have backfired spectacularly. The journal has been released to the media. Also published is a highly critical witness statement on behalf of the newspaper by Mark Bolland, the prince's former deputy private secretary, alleging that he often writes highly political letters to ministers.

Yesterday the prince's counsel, Hugh Tomlinson QC, said the prince was entitled to privacy in expressing his opinions, and that the Hong Kong journal revealed nothing politically damaging. The newspaper's lawyers argue that the public has a right to know the prince's opinions if he is trying to influence political issues.

Mr Tomlinson said there was not a "shred of evidence" that the prince sent out his journal to "influence political opinion". The Mail on Sunday was "using that argument as a cloak to appropriate the prince's rights and sell their newspapers".

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